In-depth study of moderately young but extremely red, very dusty substellar companion HD 206893B^⋆

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
² LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
³ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁵ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19c Allée du Six Août, 4000 Liège, Belgium
⁶ INAF – Catania Astrophysical Observatory, via S. Sofia 78, 95123 Catania, Italy
⁷ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁸ Max-Planck-Institute for Astronomy, Koenigsthul 17, 69117 Heidelberg, Germany
⁹ Exoplanets and Stellar Astrophysics Laboratory, Code 667, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ Aix Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
¹¹ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹³ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁴ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁵ Remote Observatory Atacama Desert (ROAD), Vereniging Voor Sterrenkunde (VVS), Oude Bleken 12, 2400 Mol, Belgium
¹⁶ Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK
¹⁷ Department of Physics & Astronomy, College of Charleston, 58 Coming Street, Charleston, SC 29424, USA
¹⁸ INAF Napoli: INAF, Astrophysical Observatory of Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁹ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
²⁰ ONERA, 29 Avenue de la Division Leclerc, 92322 Châtillon Cedex, France
²¹ NOVA Optical-Infrared Instrumentation Group at ASTRON, Oude Hoogeveensedijk 4, 7991PD Dwingeloo, The Netherlands
²² Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
²³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
²⁴ Institute for Astronomy, University of Edinburgh, Blackford Hill View, Edinburgh EH9 3HJ, UK
²⁵ CRAL, UMR 5574, CNRS, Université Lyon 1, 9 Avenue Charles André, 69561 Saint Genis Laval Cedex, France
²⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

^⋆⋆ Corresponding author: P. Delorme, e-mail: Philippe.Delorme@univ-grenoble-alpes.fr

Received: 10 May 2017
Accepted: 22 July 2017

Abstract

Context. The substellar companion HD 206893b has recently been discovered by direct imaging of its disc-bearing host star with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument.

Aims. We investigate the atypical properties of the companion, which has the reddest near-infrared colours among all known substellar objects, either orbiting a star or isolated, and we provide a comprehensive characterisation of the host star-disc-companion system.

Methods. We conducted a follow-up of the companion with adaptive optics imaging and spectro-imaging with SPHERE, and a multi-instrument follow-up of its host star. We obtain a R = 30 spectrum from 0.95 to 1.64 μm of the companion and additional photometry at 2.11 and 2.25 μm. We carried out extensive atmosphere model fitting for the companions and the host star in order to derive their age, mass, and metallicity.

Results. We found no additional companion in the system in spite of exquisite observing conditions resulting in sensitivity to 6 M_Jup (2 M_Jup) at 0.5′′ for an age of 300 Myr (50 Myr). We detect orbital motion over more than one year and characterise the possible Keplerian orbits. We constrain the age of the system to a minimum of 50 Myr and a maximum of 700 Myr, and determine that the host-star metallicity is nearly solar. The comparison of the companion spectrum and photometry to model atmospheres indicates that the companion is an extremely dusty late L dwarf, with an intermediate gravity (log g ~ 4.5–5.0) which is compatible with the independent age estimate of the system.

Conclusions. Though our best fit corresponds to a brown dwarf of 15–30 M_Jup aged 100–300 Myr, our analysis is also compatible with a range of masses and ages going from a 50 Myr 12 M_Jup planetary-mass object to a 50 M_Jup Hyades-age brown dwarf. Even though this companion is extremely red, we note that it is more probable that it has an intermediate gravity rather than the very low gravity that is often associated with very red L dwarfs. We also find that the detected companion cannot shape the observed outer debris disc, hinting that one or several additional planetary mass objects in the system might be necessary to explain the position of the disc inner edge.